Manufacture of a carbon fiber is performed by the following method, for example. Specifically, a carbon fiber precursor fiber bundle, for example, a polyacrylonitrile-based fiber bundle is transported in a multi-level manner through the inside of a heat-treatment chamber of a flameproofing furnace by folding back with rollers, heated by hot air at 200° C. to 300° C. to produce a flameproofed fiber having a desired density, before the flameproofed fiber is subjected to a carbonization treatment in a temperature range of 300° C. to 2500° C. in inert gas.
During the flameproofing, gases containing toxic materials are generated from a carbon fiber precursor fiber bundle. In order to prevent leakage of such gases from the flameproofing furnace to atmosphere, a method of forming a sealing chamber adjacent to the flameproofing furnace and lowering the pressure inside the sealing chamber compared to atmospheric pressure is known (for example, Patent Documents 1 to 4).
However, in the gases that are generated from a carbon fiber precursor fiber bundle during the flameproofing, a substance being gaseous in the heat-treatment chamber but aggregating at a lower temperature is included. In general, the temperature inside a sealing chamber is lower than the temperature inside the heat-treatment chamber. Accordingly, there would be a case in which such a substance aggregates inside the sealing chamber and adheres to a carbon fiber precursor fiber bundle. In such a case, there is a possibility of lowering strength of a carbon fiber produced by the following carbonization treatment. According to the inventions described in Patent Documents 1 to 4, such possibility is not necessarily fully considered.